Simple and cheap idea worth trying out: Piezo disk (cheap, pull one out of a piezo 
buzzer), series resistor into base of transistor. Mount to steel plate to get some 
deflection when weight applied, or direct to stone to pick up "sound" of footstep. 
Cover with silicon. Direct to stone may or may not be sensitive enough. Polarity may 
effect sensitivity and whether it operates on foot-press or foot-release. De-tune 
sensitivity by adding parallel R.

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Brent Brown, Electronic Design Solutions
16 English Street, St Andrews,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: 027 433 4069
eMail:  brent.brown@clear.net.nz


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